Thermocouple vacuum gauges are known in the art. Typically, such gauges employ one or more thermocouples in a circuit configuration.
A thermocouple is, itself, an electric circuit consisting of a pair of wires of dissimilar metals joined together at one end (the sensing or measuring junction, also called the hot junction) and terminated at their other end in such a manner that the terminals (reference junction or cold junction) are both at the same and known temperature (reference temperature). Leads are connected from the reference junction to some sort of load resistance (e.g., an indicating meter or the input impedance of other readout or signalconditioning equipment) to complete the thermocouple circuit. Both these connecting leads can be of copper or some other metals dissimilar from the metals joined at the sensing junction.
Due to the thermoelectric effect (Seebeck effect), a current is caused to flow through the circuit whenever the sensing junction and the reference junction are at different temperatures. The electromotive force (thermoelectric EMF) which causes current to flow through the circuit is dependent in its magnitude on the sensing junction wire materials, as well as on the temperature difference between the two junctions.
Thermocouple vacuum gauges are widely used as vacuum measuring devices in the pressure range of 10.sup.-3 to 1 torr. The term "vacuum" which strictly speaking implies the unrealizable ideal of space entirely devoid of matter, is used in a relative sense to denote gas pressure below the normal atmospheric pressure of 760 torr. The degree or quality of the vacuum attained is indicated by the total pressure of the residual gases in the vessel which is pumped. By standard conventional terminology, a coarse or rough vacuum has a pressure range from 760 to 1 torr, a medium vacuum has a pressure range from 1 to 10.sup.-3 torr, a high vacuum has a pressure range from 10.sup.-3 to 10.sup.-7 torr, a very high vacuum has a pressure range from 10.sup.-7 to 10.sup.-9 torr, and an ultrahigh vacuum has pressures smaller than 10.sup.-9 torr.
In a typical thermocouple vacuum gauge, a thermocouple is heated either directly (e.g., by sending current through it) or indirectly (e.g., by a separate heater). The EMF generated by the temperature gradient between the reference or cold junction and the sensing or hot junction is recorded as a measure of the sensing or hot junction temperature. The sensing or hot junction temperature will change with the pressure of the surrounding gas because of heat transfer from the hot wire. The variation with pressure of the heat transfer from a hot wire through the surrounding gas is shown schematically in prior art FIG. 1. In the low pressure region, the heat transfer by conduction through the surrounding gas is proportional to the log of gas density or pressure. In the higher pressure regions (above approximately 2 torr) and in the absence of natural or forced convective cooling, the thermal conductivity is virtually independent of pressure. This independent relationship renders thermocouple devices useless for measuring pressure in these regions. If heat transfer by convection is encouraged in these higher pressure regions, it is possible to obtain additional pressure dependent heat transfer. The broken line in FIG. 1 represents such an extension of this proportional relationship, thereby allowing one to extend upward the range of pressures that can be measured by thermal conductivity gauges.
Various attempts in the prior art to extend the range of thermal conductivity gauges by forced (active) and natural convection are described in Steckelmacher, W. "The high pressure sensitivity extension of thermal conductivity gauges," Vacuum, vol. 23, no. 9, Pergamon Press Ltd., 1973, 307-311.
U.S. Pat. No. 4,579,002 discloses a thermocouple vacuum gauge for measuring pressure in an evacuated enclosure. FIG. 1 of that patent shows a block diagram of the gauge. A time-multiplexed servomechanism 10 is used to supply a duration modulated constant amplitude heating pulse to thermocouple 12. In the intervals between heating, the EMF of the thermocouple is measured and compared to a reference voltage. The current needed to maintain the thermocouple at a constant temperature determines the duty cycle of the pulses. This duty cycle is a function of the pressure in the apparatus. Only one thermocouple is employed in this system. A linear response of temperature vs. pressure over approximately six orders of magnitude of pressure is possible with this configuration. A similar type of device is described in U.S. Pat. No. 4,633,717.
Thermocouples are also used for measuring temperature, pressure and fluid flow. Employing a plurality of thermocouples in a single measuring circuit is known in the art.
U.S. Pat. No. 3,030,806 discloses a flowmeter which utilizes temperature-difference effects manifested in Seebeck-Peltier junctions in response to a change in physical conditions (i.e., fluid flow). In the embodiment depicted by FIG. 3 of this patent, thermocouples S1 and S2 are connected in series with each other and disposed within the path of fluid flow. Thermocouple S1 has a Seebeck junction and thermocouple S2 has a Peltier junction. An A.C. energization source 150 is connected to thermocouple S1. During one-half of the A.C. energization source cycle, the Peltier junction will be heated above ambient temperature and the flow of fluid passing the Peltier junction will result in a cooling effect. Similarly, on the following half of the A.C. energization source cycle, the Peltier junction will be cooler than ambient temperature and will, therefore, absorb heat from the fluid medium being measured. The temperature changes will produce a varying current flow in the Seebeck thermocouple. The amplitude of the current flow will be decreased due either to such heating or cooling by the fluid flow medium. The resultant output signal of the series connected thermocouples is, therefore, an alternating current signal or modulated carrier signal. The degree of modulation represents, or is proportional to, the rate of fluid flow passing the junctions. In the FIG. 3 embodiment of this patent, thermocouples S1 and S2 are continuously connected in series.
U.S. Pat. No. 3,903,743 discloses a typical temperature compensating circuit where one of two series connected and oppositely poled thermocouples is employed for correction. None of the thermocouples are alternately heated.
U.S. Pat. No. 2,745,283 discloses another temperature compensation circuit for a thermal measuring device which employs thermocouples. In the FIG. 4 embodiment of U.S. Pat. No. 2,745,283, the compensating thermocouple is not connected in series with the heated thermopile (battery of thermocouples).
U.S. Pat. No. 4,492,123 discloses probe-type thermal conductivity vacuum gauges (Pirani gauges) which are insensitive to the physical and electrical nature of the probe circuitry. The invention allows for the transmission of low measuring voltages from the thermocouples to be unaffected by long lead lengths or circuit elements, thereby improving the accuracy of the gauges.
Many prior art designs use a very high temperature filament to extend the range of thermal conductivity gauges. However, high temperatures are undesirable for most applications. Furthermore, prior art thermocouple gauges, even those which employ a constant sensing element temperature rather than a constant heating current, are limited in sensitivity (accuracy) at high pressure.
In spite of the extensive efforts in the prior art to extend the operating range of thermocouple vacuum gauges, there is still a need for a thermocouple vacuum gauge which has an extended range without the need to employ high operating temperatures, varying temperatures or forced (active) convective apparatus. There is also still a need for a vacuum gauge which has improved accuracy in the high pressure range. There is further a need for achieving these goals through a design that is inexpensive and simple to fabricate. The present invention fills those needs.